Huygens principle in action - reflection on a parabola

This applet shows Huygens principle in action. In the animation, (blue) waves are generated in the focus of a parabola and on reflecting on it (green) secondary wavelets are produced. The envelopes of the secondary wavelets are, as expected, plain waves. You can adjust the shape of the parabola by acting on its upmost point, and choose wavelength, number of waves and number of wavelets by acting on the sliders.